new question added

Author: arorarahul

README.md

@@ -92,6 +92,8 @@ TODO:
 - [Find the subarray with the given sum](/arrays/question20.c) 
 - [Consider and array which contains only 0's and 1's. Find largest sub array which contains only 0's and 1's](/arrays/question21.c) 
 - [Given an array of n-integers, construct product array such that prod[i] is equal to product of all elements except arr[i] without using division operator](/arrays/question22.c)
+- [Find the duplicate in O(n) time and O(1) extra space](/arrays/question23.c)
+
 
 ## Some important concepts to solve algos better
 

arrays/question23.c

@@ -0,0 +1,26 @@
+/*
+Find the duplicate in O(n) time and O(1) extra space
+Only valid for numbers in range 1 to N
+
+Method: In this we visit a number lets say its 3, we go to index 3 and make the number there
+negative. Then if 3 is repeated we will again go to 3 and check if its already negative.
+Time complexity: O(n)
+Space complexity: O(1)
+*/
+
+#include <stdio.h>
+#include <stdlib.h>
+
+int main(){
+
+	int a[] = {1,2,3,5,8,1,1,2};
+	int size = sizeof(a)/sizeof(a[0]);
+	
+	for(int i=0;i<size;i++){
+		if(a[abs(a[i])] > 0){
+			a[abs(a[i])] = -1*a[abs(a[i])];	
+		}else{
+			printf("%d\n", abs(a[i]));
+		}
+	}
+}